allEqual and allDistinct

[DmitryNekrasov]

This is a place to discuss the allEqual and allDistinct proposal. The text of the proposal is located here.

The goal of the proposal is to add two families of short-circuiting extension functions: allEqual() / allEqualBy { } check whether all elements of a collection are equal to each other, and allDistinct() / allDistinctBy { } check whether they are all different from each other.

Feedback is especially welcome on the two decisions that keep the API experimental: the floating-point equality model and the absence of a custom-predicate overload.